Magnetic resonance imaging (MRI) is commonly used to image the internal tissues of a subject. Magnetic resonance elastography (MRE) is a technique for determining mechanical properties of a subject under study by introducing mechanical vibrations in the subject undergoing MRI.
MRI is typically performed by placing the subject or object to be imaged at or near the isocenter of a strong, uniform magnetic field, B0, known as the main magnetic field. The main magnetic field causes the atomic nuclei (spins) that possess a magnetic moment in the matter comprising the subject or object to become aligned in the magnetic field. The spins form a magnetization that precesses around the magnetic field direction at a rate proportional to the magnetic field strength. For hydrogen nuclei (which are the common nuclei employed in MRI), the precession frequency is approximately 64 MHz in a magnetic field of 1.5 Tesla. If the magnetization is perturbed by a small radio-frequency magnetic field, known as a B1 magnetic field, the spins emit radiation at a characteristic radio frequency (RF). The emitted RF radiation can be detected and analyzed to yield information that may be used to produce an image of the subject or object. For purposes of the discussion herein, the term “object” will be used to refer to either a subject (e.g., a person) or an object (e.g., a test object) when describing magnetic resonance imaging of that “object.”
In practice, magnetic field gradients are also applied to the subject or object in addition to the main magnetic field. The field gradients are typically applied along one or more orthogonal axes, (x, y, z), the z-axis usually being aligned with the B0, and introduce spatially-distributed variations in frequency and/or phase of the precessing nuclear spins. By applying the radio-frequency B1 magnetic field and gradient fields in carefully devised pulses and/or sequences of pulses that are switched on and off, the RF radiation emitted can carry spatially encoded information that, when detected and analyzed, can be used to produce detailed, high resolution images of the subject or object. Various techniques utilizing both specific pulse sequences and advanced image reconstruction methods have been developed, providing new advances, as well as introducing new challenges.
An MRI system typically includes hardware components, including a plurality of gradient coils positioned about a bore of a magnet, an RF transceiver system, and an RF switch controlled by a pulse module to transmit RF signals to and receive RF signals from an RF coil assembly. The received RF signals are also known as magnetic resonance (MR) signal data. An MRI system also typically includes a computer programmed to cause the system to apply to an object in the system various RF signals, magnetic fields, and field gradients for inducing spin excitations and spatial encoding in an object, to acquire MR signal data from the object, to process the MR signal data, and to construct an MR image of the object from the processed MR signal data. The computer can include one or more general or special purpose processors, one or more forms of memory, and one or more hardware and/or software interfaces for interacting with and/or controlling other hardware components of the MRI system.
MR signal data detected from an object are typically described in mathematical terms as “k-space” data (k-space is the Fourier inverse of image or actual space). An image in actual space is produced by a Fourier transform of the k-space data. MR signal data are acquired by traversing k-space over the course of applying to the object the various RF pulses and magnetic field gradients. In practice, techniques for acquiring MR signal data from an object are closely related to techniques for applying the various RF pulses and magnetic field gradients to the object.
In MRE, external vibrations are introduced into an object, such as biologic tissue, under examination. Vibrations in the tissue (or object) are encoded in the MR signal phase using standard MRI sequences upgraded with motion encoding gradients (MEG). As a result, tissue mechanical parameters can be calculated from the acquired wave fields. In the study of live subjects, the analysis of MRE data with one motion-encoding direction can reveal a correlation of pathophysiological changes and the mechanical behavior of diverse organs. However, difficulties in resolving early disease stages have also become apparent. A step towards a higher diagnostic accuracy is represented by the acquisition of the three-dimensional (3D) displacement field, by which it becomes possible to separate shear from a compression wave by using the curl-operator, and which sets asides any assumptions about the direction of wave propagation in the wave field inversion algorithm.
In MRE, measurement time may be of interest. Aside from cost factors, long acquisition times may have the potential of decreasing measurement accuracy, since motion may occur and cause misalignment of the images. Further, in conventional 3D MRE, the components of the tissue (or object) displacement are acquired in three individual temporally resolved MRE experiments carried out sequentially in time. Therefore, the components, although attributed to the same point in time, are actually acquired in different physiological states of the subject.
Accordingly, there is a need to improve the method for imaging and analyzing tissue samples using MRE techniques.